Removal of heat from electronic circuitry has become one of the limiting factors in the design and performance of most computer systems and many other electronic devices. Power dissipation of a computer increases approximately as the operating frequency squared. Thus, doubling the clock speed, roughly doubling computer system performance, will require nearly four times the power dissipation. Without further innovation in the area of heat removal the development of a next generation computer design will be hampered.
One of very few practical ways to remove heat generated in the processing modules of very high-speed high-performance computers (supercomputers) is by spraying a thin layer of liquid directly onto the computer chips. Heat is then transported from the surface by heating the flowing liquid and by boiling off some fraction of the liquid (two phase cooling). This method of heat removal is known as spray cooling, or spray evaporative cooling, and is a very efficient method of removing very high heat fluxes from small surfaces. Thus, spray evaporative cooling is growing in prominence and application in the cooling of electronics and laser diodes because of the need for high heat transfer in a small area in such applications.
The physical mechanisms of spray evaporative cooling are not completely understood. However, current research based on single nozzle spray systems suggests that less than 50% of the heat is removed through nucleate boiling, similar to boiling a pot of water on a stove. More than 50% of the heat is removed either directly by heating the flowing liquid film or by evaporation at the surface of the film. It also has been shown that the heat flux increases both with the area of the heated surface directly covered by the spray and by the flux of spray droplets impinging on the surface.
Current spray cooling systems and methods operate by spraying a dielectric fluid, e.g., Fluorinert 72 (FC-72), at a normal or other angle directly onto computer chips, from either above or below the chips, using a cone shaped spray. The objective is to create a thin film of cooling fluid on the surface to be cooled to remove heat through single phase and two phase convection. While such systems are adequate for most current computer systems and other electronic applications, the ability of such systems to remove heat has been pushed to its limits. For example, in such systems multiple nozzles often are used to direct multiple cone shaped cooling fluid sprays onto a surface to be cooled to increase the total spray droplet flux impinging on the surface. However, in such systems the plumes of fluid that are sprayed from the nozzles tend to interact to create pockets of relatively low cooling fluid momentum, where the fluid tends to build up and where boiling often occurs. During the cooling process bubbles are generated that cause greatly enhanced mixing so that the liquid surface temperature is raised to the point that a large amount of evaporation occurs there. Stagnation regions will stop the mechanical mixing, leading to bubble growth and dry out at the surface being cooled. Thus, the interaction of the sprays leads to inefficient cooling and liquid build-ups on the heated surface, creating regions of poor heat transfer and, therefore, non-uniform heat transfer across the surface of the chip. In the regions of poor heat transfer, the surface temperatures can rise significantly above the average surface temperature, causing the surface temperature to “run away”, leading to catastrophic failure.
For optimal operation, it also is imperative that the coolant be removed from the spray region quickly and efficiently to prevent the buildup of warm liquid. Failures also can occur if the coolant liquid is not efficiently removed from the surface being cooled. Current spray cooling systems are limited in the ability to drain spray coolant after impacting a heated surface. In particular, in many such systems changing the orientation of the system even slightly will affect coolant drainage patterns, thereby adversely affecting the cooling ability of the system. This limits the application of current spray cooling systems in many applications where the system to be cooled is portable and thus subject to changes in orientation.
Thus, current spray cooling systems suffer from various limitations including low critical heat flux (burnout) conditions, non-uniform heat transfer over large areas (area coverage limitations), and orientation constraints due to draining patterns.
What is desired, therefore, is an improved evaporative spray cooling system and method in which a coolant is provided onto a surface to be cooled so as to maintain a uniform thin layer of coolant thereon, avoiding the creation of areas of interaction between coolant sprays from different spray nozzles that can result in coolant build up, poor heat transfer, and possible circuit failure. What also is desired is an improved evaporative spray cooling system and method in which the uniform thin layer of coolant and adequate coolant drainage are maintained despite variations in orientation of the system being cooled thereby.